There is strong market pressure to reduce the cost of integrated circuits (ICs). These cost considerations are causing IC manufacturers to use smaller feature sizes within the ICs, which permits a greater density of features on the die. As a result, the likelihood is increasing that a given manufacturing defect will impinge on one of these features and produce a malfunctioning IC.
At the same time, there is strong motivation to provide ever more functionality, and thus to include larger and larger numbers of transistors in an IC, resulting in a larger die. All else being equal, the likelihood of a manufacturing defect occurring on a die increases rapidly as the die area increases. Thus, a larger die size means a much greater probability of a manufacturing defect affecting each die.
One well-known method of addressing these limitations is to manufacture several smaller dice and incorporate them together into a single packaged product known as a “multi-chip module”, or MCM. Because several smaller dice are used, each die has a much lower probability of including a manufacturing defect. Further, each die can be at least minimally tested prior to inclusion in the MCM, which further increases the final product yield. Using an MCM also allows the combination of several die from different sources.
An MCM includes a substrate, commonly made of a ceramic material, on which are mounted two or more dice, flipped so that the input/output (I/O) pads of the die can be coupled to lands on the substrate. The combined substrate and dice are then packaged in a fashion similar to that used to package single ICs.
FIG. 1 shows a side view of an MCM that includes a substrate 101 and two IC dice 102, 103. Solder bumps 104 couple the die I/O pads to the substrate 101 via substrate lands 105. Substrate 101 includes metallic interconnect lines (not shown in FIG. 1) that interconnect some of the lands 105 according to a predetermined pattern, thus coupling the I/O pads of the dice to each other. Others of the substrate lands are coupled to package pins of the MCM package using well-known techniques such as wire bonding via wires 106, 107.
FIG. 2 shows a cutaway view of substrate 101 of FIG. 1 and its associated lands. Substrate 101 is a layered construct manufactured using techniques similar to those used to manufacture printed circuit boards (PCBs). Layers of dielectric material 208 (shown cross-hatched in FIG. 1) isolate metallic traces 209 from each other. Conductive vias 210 perpendicular to the surface of the substrate interconnect the lands 105 with the conductive traces 209.
By laying out the conductive traces in a predetermined fashion, the desired interconnections are implemented among the die I/O pads and between the die I/O pads and the MCM package pins. For example, in the MCM substrate 101 shown in FIGS. 1 and 2, land 105a is coupled through a bonding wire 106 to a package pin, and also coupled through via 210a, trace 209a, and via 210b to land 105b. Note that, as in a PCB layout, the interconnections between the various die I/O pads are fixed and cannot be changed without retooling and manufacturing a new substrate 101.
This inflexibility has been addressed in various ways. For example, a type of programmability has been provided by “programming” the substrate using wire bonding, or by cutting traces and using jumpers to make the new connections. Another known method of providing flexibility is to use a dual-sided substrate that carries multiple prepackaged PLDs on a top side and a field programmable interconnect (FPIC) die on a bottom side. Thus, programmable connections are provided within the FPIC die, and not by the substrate itself.
It has also been proposed that programmable junctions be provided between metal traces, the junctions comprising amorphous silicon antifuses that are normally insulators, but are selectively programmable after the substrate is manufactured by applying a voltage pulse across the antifuse to render it conductive. The drawback to this technique is that the programming of the substrate is irreversible.
While some degree of flexibility is provided by these techniques, known MCMs either lack the ability to alter the interconnect pattern after programming, or require additional dice to provide the programming capability. Therefore, it is desirable to provide an MCM that offers a heightened degree of flexibility without requiring that additional die be added to the MCM.
Further, when known MCM substrates are used, all electrical functions are performed within the IC dice mounted to the MCM substrate. For example, as MCMs increase in size, the MCM can become large enough to make buffering of signals on long traces a desirable feature. Using known MCM substrates, any such buffers are implemented either within one of the dice already included in the MCM or by adding another die to perform the buffering function. Therefore, it is desirable to provide an MCM substrate that can perform common electrical functions without using these resources.